Electrical high-voltage insulators are made of glass, porcelain or plastic. They have a long service life when exposed to normal environmental influences. If they are exposed to severe environmental influences, i.e., high contaminating loads, such as are due, for example, to dust and dirt in an industrial atmosphere, to salt mist in a coastal region or to dust and erosion in a desert atmosphere, these conducting impurities result in flashovers and consequently in a shortening of the service life. It is, therefore, necessary to clean such insulators frequently. Therefore, in order to avoid the need for frequent cleaning of the electrical high-voltage insulators, they are, therefore, coated with organopolysiloxane elastomers which hydrophobize the contaminating loads.
U.S. Pat. No. 4,476,155 to Niemi describes compositions which are suitable for coating the surface of high-voltage insulators, are stable on storage in the absence of water and can be crosslinked to form elastomers at room temperature when exposed to water. These compositions are prepared by mixing a dimethylpolysiloxane having terminal groups capable of condensation with a silane having at least three oxime groups bonded to a silicon atom via oxygen, aluminum hydroxide having an average particle size of less than 5 .mu.m and, optionally, a nonreactive solvent. Niemi discloses using aluminum hydroxide in amounts of from 35 to 55 percent by weight, based on the total weight of the composition composed of dimethylpolysiloxane, silane and aluminum hydroxide.
Therefore, it is an object of the present invention to provide compositions which are stable on storage in the absence of water and can be crosslinked to form elastomers at room temperature when exposed to water and when applied on electrical high-voltage insulators, provide thin coatings which have a good to better creep resistance and arc resistance and better mechanical properties, such as, tensile strength and elongation at break, than the compositions known heretofore. Another object of the present invention is to provide coating compositions for electrical high-voltage insulators having mechanical properties which can be altered in a simple manner. Still another object of the present invention is to provide compositions which are stable on storage in the absence of water and can be crosslinked to form elastomers at room temperature when exposed to water and which, compared with compositions having the same viscosity, yield uniform and continuous coatings when sprayed onto the surface of electrical high-voltage insulators and have only a slight tendency to form drops or runs at the edges of the insulator disks. A further object of the present invention is to provide compositions which are stable on storage in the absence of water and can be crosslinked to form elastomers at room temperature when exposed to water and when applied as a coating to electrical high-voltage insulators provide insulators having a long service life, especially in regions in which they are exposed to severe contamination and moisture.